Gas turbine engines are well known sources of power, e.g., motive power for aircraft or as power generators, and generally include compressor (typically preceded by one or more fan stages), combustor and turbine sections. FIG. 1 schematically illustrates a gas turbine engine 20. The gas turbine engine 20 is disclosed herein as a two-spool turbofan that generally incorporates a fan section 22, a compressor section 24, a combustor section 26 and a turbine section 28. Alternative engines might include an augmentor section (not shown) among other systems or features. The fan section 22 drives air along a bypass flow path B in a bypass duct, while the compressor section 24 drives air along a core flow path C for compression and communication into the combustor section 26 then expansion through the turbine section 28. Although depicted as a two-spool turbofan gas turbine engine in the disclosed non-limiting embodiment, it should be understood that the concepts described herein are not limited to use with two-spool turbofans as the teachings may be applied to other types of turbine engines including three-spool architectures.
The exemplary engine 20 generally includes a low speed spool 30 and a high speed spool 32 mounted for rotation about an engine central longitudinal axis A relative to an engine static structure 36 via several bearing systems 38. It should be understood that various bearing systems 38 at various locations may alternatively or additionally be provided, and the location of bearing systems 38 may be varied as appropriate to the application.
The low speed spool 30 generally includes an inner shaft 40 that interconnects a fan 42, a low pressure compressor 44 and a low pressure turbine 46. The inner shaft 40 is connected to the fan 42 through a speed change mechanism, which in exemplary gas turbine engine 20 is illustrated as a geared architecture 48 to drive the fan 42 at a lower speed than the low speed spool 30. The high speed spool 32 includes an outer shaft 50 that interconnects a high pressure compressor 52 and high pressure turbine 54. A combustor 56 is arranged in exemplary gas turbine 20 between the high pressure compressor 52 and the high pressure turbine 54. An engine static structure 36 is arranged generally between the high pressure turbine 54 and the low pressure turbine 46. The engine static structure 36 further supports bearing systems 38 in the turbine section 28. The inner shaft 40 and the outer shaft 50 are concentric and rotate via bearing systems 38 about the engine central longitudinal axis A which is collinear with their longitudinal axes.
The core airflow is compressed by the low pressure compressor 44 then the high pressure compressor 52, mixed and burned with fuel in the combustor 56, then expanded over the high pressure turbine 54 and low pressure turbine 46. The turbines 46, 54 rotationally drive the respective low speed spool 30 and high speed spool 32 in response to the expansion. It will be appreciated that each of the positions of the fan section 22, compressor section 24, combustor section 26, turbine section 28, and fan drive gear system 48 may be varied. For example, gear system 48 may be located aft of combustor section 26 or even aft of turbine section 28, and fan section 22 may be positioned forward or aft of the location of gear system 48.
The engine 20 in one example is a high-bypass geared aircraft engine. In a further example, the engine 20 bypass ratio is greater than about six (6), with an example embodiment being greater than about ten (10), the geared architecture 48 is an epicyclic gear train, such as a planetary gear system or other gear system, with a gear reduction ratio of greater than about 2.3 and the low pressure turbine 46 has a pressure ratio that is greater than about five. In one disclosed embodiment, the engine 20 bypass ratio is greater than about ten (10:1), the fan diameter is significantly larger than that of the low pressure compressor 44, and the low pressure turbine 46 has a pressure ratio that is greater than about five 5:1. Low pressure turbine 46 pressure ratio is pressure measured prior to inlet of low pressure turbine 46 as related to the pressure at the outlet of the low pressure turbine 46 prior to an exhaust nozzle. The geared architecture 48 may be an epicycle gear train, such as a planetary gear system or other gear system, with a gear reduction ratio of greater than about 2.3:1. It should be understood, however, that the above parameters are only exemplary of one embodiment of a geared architecture engine and that the present invention is applicable to other gas turbine engines including direct drive turbofans.
A significant amount of thrust is provided by the bypass flow B due to the high bypass ratio. The fan section 22 of the engine 20 is designed for a particular flight condition—typically cruise at about 0.8 Mach and about 35,000 feet (10,688 meters). The flight condition of 0.8 Mach and 35,000 ft (10,688 meters), with the engine at its best fuel consumption—also known as “bucket cruise Thrust Specific Fuel Consumption (‘TSFC’)”—is the industry standard parameter of lbm of fuel being burned divided by lbf of thrust the engine produces at that minimum point. “Low fan pressure ratio” is the pressure ratio across the fan blade alone, without a Fan Exit Guide Vane (“FEGV”) system. The low fan pressure ratio as disclosed herein according to one non-limiting embodiment is less than about 1.45. “Low corrected fan tip speed” is the actual fan tip speed in ft/sec divided by an industry standard temperature correction of [(Tram °R)/(518.7°R)]0.5. The “Low corrected fan tip speed” as disclosed herein according to one non-limiting embodiment is less than about 1150 ft/second (350.5 m/sec).
As illustrated generally in FIG. 2, compressor section 24 and turbine section 28 (and any fan stages 22) each include shaft-mounted, rotating disks 1, each carrying a set of blades 2 located within static structure 36, with intervening sets of stationary vanes 5 mounted to the static structure 36. Air seals 4, 7 are provided between the tips of the blades 2 and the static structure 36 (outer air seals), and between the free ends 6 of the vanes 5 and the knife edges 8 of the disks 1 (knife edge seals) to prevent air leakage between those components.
Air is ingested through the core flow path C and compressed by rotating disks 1 and associated blades 2 in the compressor section 24. The compressed air is then burned with fuel in the combustor section 26 to generate high pressure and temperature gasses, which cause rotation of the turbine section 28 and associated fan section 22 and compressor section 24 and are then ejected out an engine exhaust to provide thrust. The static structure 36 is intended to prevent leakage of air or combustion products around the tips of the blades 2, i.e., between the blade 2 tips and the static structure 36, which leakage reduces the efficiency of the engine 20.
Despite the design of components to minimize leakage, a substantial proportion of any leakage which does occur in a normally-operating gas turbine engine occurs between the tips of the blades 2 and the static structure 36, and between the tips of the vanes 5 and the disks 1. One manner of eliminating such leakage is to fabricate all mating parts to extremely close tolerances, which becomes increasingly expensive as tolerances are reduced. Moreover, given the temperature ranges to which the parts are subjected to before, during and after operation, and the resultant thermal expansion and contraction of the parts, such close tolerances will at times result in interference between mating parts and corresponding component wear and other damage. Accordingly, gas turbine engine designers have devoted significant effort to developing effective air seals, and particularly seals composed of abradable materials. Such seals require a balance of several properties including abradability upon being contacted by a rotating blade 2 tip, erosion resistance, durability, thermal expansion balanced with that of the underlying material, and relative ease and reasonable cost of manufacture.
A typical compressor air seal includes a seal substrate, e.g., a metal substrate, a metal bond layer composed of a metal powder plasma sprayed onto the substrate, and an abradable, sealing layer which is also typically plasma sprayed onto the metal bond layer. A typical sealing layer includes a metal matrix of aluminum and silicon with some amount of embedded methyl methacrylate powder particles. Because a galvanic cell is formed between the aluminum and silicon, the abradable sealing layer is subject to environmental aqueous corrosion, thereby possibly compromising the performance of the sealing layer.
Improvements in seal design are therefore needed in the art.